The present invention relates to wireless transmission and in particular to non-linear transmitters which use predistortion.
FIG. 1 is a block diagram of a typical radio frequency transmitter 10, used in mobile phones. Generally, radio frequency (RF) transmitter 10 comprises a power amplifier (PA) 12 which amplifies the signals being transmitted from an antenna 14. For efficiency, amplifier 12 is, in many cases, a non-linear amplifier operated near its peak capacity. To avoid distortion of the transmitted signals due to the non-linearity, the signals are pre-distorted by a predistorter 15 before they are transmitted. The predistortion is required to prevent transmitter 10 from transmitting signals on channel bands other than the band assigned to the transmitter. A multiplier 16 of predistorter 15 multiplies modulated signals prepared for transmission by predistortion values. The predistortion values are chosen such that the product values entering amplifier 12 will be distorted by the power amplifier to return to a substantially linear amplification of the modulated signals. The predistortion values are generally selected from a look up table (LUT) 18 which is indexed by the amplitude of the multiplied transmitted signal. As the exact predistortion caused by the amplifier varies with time and carrier frequency, due to temperature, aging and other characteristics of power amplifier 12, the predistortion values in LUT 18 are updated, by a trainer 20, based on feedback received from the output of amplifier 12.
A paper titled xe2x80x9cAmplifier Linearization Using a Digital Predistorter with Fast Adaptation and Low Memory Requirementsxe2x80x9d, J. K. Cavers, IEEE Trans. On Vehicular Technology, Vol. 39, No. 4, Nov. 1990, and U.S. Pat. No. 5,049,832 to Cavers, describe an adaptive method for calculating the predistortion values of LUT 18. In this method, LUT 18 is assigned initial random values which are thereafter updated adaptively. For each sample provided to predistorter 15 and thereafter to power amplifier 12, the difference between the desired amplification and the actual amplification of the power amplifier is calculated and accordingly the corresponding entry in LUT 18 is adjusted. This method, however, does not converge fast enough to fulfill requirements of emerging standards.
U.S. Pat. No. 5,923,712 to Leyendecker et al., describes another method for calculating the predistortion values of LUT 18. In the method of the U.S. Pat. No. 5,923,712 patent, trainer 20 accumulates a plurality of sets of three values including the modulated value being provided to predistorter 15, the value after it is pre-distorted (as it enters to power amplifier 12) and the value at the output of power amplifier 12. Periodically trainer 20 divides the recently accumulated sets into bins which correspond to the entries of LUT 18. These entries of LUT 18 are indexed using the values provided to predistorter 15. Using the accumulated values of each bin, trainer 20 directly determines an inverse transfer function of the power amplifier for the specific bin. The inverse transfer function is determined by calculating the transfer function of power amplifier 12 from its output to its input. According to the inverse transfer function of the specific bin the corresponding entry of LUT 18 is updated. Bins which have too few samples are not used for updating the entries of LUT 18, and instead interpolation is performed using neighboring bins.
The method of the U.S. Pat. No. 5,923,712 patent, however, does not operate fast enough and does not reach high enough levels of accuracy, for example, due to impairments of analog components of the transmitter. In order to compensate for inaccuracies, the U.S. Pat. No. 5,923,712 patent suggests using previous sample information in determining which entry of the LUT 18 is to be used and in updating the LUT 18. The use of the previous sample information, by the trainer 20 of the U.S. Pat. No. 5,923,712 patent, complicates the trainer, increases the size of its memory buffer and enlarges its current consumption.